1. Field of the Invention
The present invention relates to heteropolysaccharides produced by bacteria of the genus Xanthomonas and to methods for modifying such heteropolysaccharides. The invention further relates to an improved method for recovering oil from a subterranean oil-bearing formation.
2. Description of the Prior Art
Heteropolysaccharides produced by the action of bacteria of the genus Xanthomonas are used in many applications. They are used in many foodstuffs and beverages. They are used as suspending agents in many cosmetic and pharmaceutical preparations. For these uses a bipolymer which will form solutions of high clarity or which will not undesirably discolor the preparations is needed. Biopolymers are also used as a viscosifier for water which is injected into oil-bearing subterranean formations. This viscous water will more readily displace oil from the formation in most instances than will water which does not contain the polymer. Because of the need for a readily injectable biopolymer for oil recovery purposes, the remainder of this discussion of the prior art will be directed to general problems of recovering oil from subterranean formations, of using materials to increase the viscosity of water used to displace oil from such reservoirs, and of producing a viscosifier which can be more readily injected into such formations.
Recognition that techniques utilized in the initial exploitation of subterranean oil-bearing reservoirs generally permit the recovery of only a small fraction of the total oil originally present in such reservoirs has led to the development of a number of secondary and tertiary (enhanced) recovery processes designed to stimulate production after the natural energy of the reservoir has been largely expended. The most widely used of these is the wateflooding process. By simply injecting water into an underground reservoir through one or more injection wells under sufficient pressure to force it in the direction of production wells spaced some distance from the injection wells, much of the oil left in the reservoir after the wells have ceased to flow at an economical rate can be recovered. The waterflooding process is considerably more attractive than many other enhanced recovery processes of the displacement type because the water utilized can ordinarily be obtained at little cost and need not be recovered from the reservoir in order to make the process economically feasible.
Despite the obvious advantages of waterflooding as an enhanced recovery technique, it has often suffered by comparison with other processes. Field evaluations have shown that waterflooding permits the recovery of much oil that cannot be produced by primary recovery techniques but that considerable quantities of oil nevertheless remain in the reservoir following a waterflooding operation. The principal reason for this appears to be the tendency of the injected water to finger through the sections of the reservoir offering the least resistance and to thus bypass much of the oil present in the reservoir. In addition, it has been demonstrated that capillary and surface forces prevent the displacement of appreciable quantities of the oil present in those sections of the reservoir through which the water actually passes.
Fundamental investigations into the mechanisms by which one fluid displaces another within a porous medium have demonstrated that the relative viscosities of the two fluids play an important role in determining the efficiency of the displacement. It can be shown that the displacement efficiency is directly related to the ratio of viscosity of the displacing fluid to that of the displaced fluid. Petroleum normally ranges in viscosity from one to two centipoises up to about 1,000 centipoises or more, depending upon the particular oil reservoir in which it is found. The viscosity of water, on the other hand, is less than one centipoise under the conditions prevailing in most subterranean oilbearing reservoirs. The ratio of the viscosity of water to that of oil is therefore low, and hence high displacement efficiencies during enhanced recovery processes using water as the displacing agent are not to be expected.
In view of this effect of the viscosity ratio upon displacement efficiency, it has been suggested that a more viscous liquid than water be employed as a displacing agent during enhanced recovery operations. Economic considerations dictate that the liquid so used be an aqueous solution. A variety of polymeric materials and other thickening agents have been advocated as useful for preparing such solutions. Tests have demonstrated that many of the materials proposed heretofore are not usable, even though they can be employed to form relatively viscous aqueous solutions, because they lack other properties essential to a satisfactory thickener for enhanced recovery purposes. Some of the materials proposed in the past are relatively expensive and must be used in concentrations which make the cost prohibitive. Moreover, solutions of many such materials tend to plug the pore spaces of the permeable rock which makes up most oil reservoirs and hence would be unsatisfactory even if their use were economically feasible. Other materials are decomposed, seriously degraded, or precipitated at the temperature prevailing in many subsurface reservoirs and by contact with reservoir sands and connate waters. Still other materials are adsorbed onto the rock surfaces within the reservoirs to such an extent that any increase in viscosity attained is lost almost immediately following injection of solutions of the materials into an injection well.
Considerable interest in microbially produced polysaccharides has been exhibited in recent years. Impetus has been given to the development of this interest by the discovery that certain polysaccharides formed by biochemical synthesis have properties which permit their use as thickening agents for water used in enhanced recovery operations carried out in the petroleum industry. It has been found that some of these materials added to water or brine in suitable concentrations produce viscous solutions which are relatively stable under the conditions which previal in subsurface oil reservoirs. By utilizing a solution of controlled viscosity in place of the water or brine normally employed in waterflooding projects, a more favorable mobility ratio can be obtained between the oil in the reservoir and the liquid used to displace it. The tendency of the displacing medium to finger through highly permeable sections of the reservoir without displacing oil from the less permeable sections is greatly decreased. Viscous forces which normally reduce the displacement efficiency in portions of the reservoir through which the displacing medium actually passes are more readily overcome. As a result of these effects, the use of water or brine containing polysaccharide thickening agents generally permits the recovery of significantly greater quantities of oil during waterflooding than can be recovered with water or bine alone.
In U.S. Pat. No. 3,305,016 to Lindblom et al. there is disclosed a particularly effective polysaccharide for use as a thickening agent during oil field waterflooding operations, i.e., the heteropolysaccharide produced by the action of bacteria of the genus Xanthomonas upon sugar, starches, other carbohydrates, methanol, ethanol, and acetates. Studies and comparative tests have shown that this material, a polymer generally containing mannose, glucose, glucuronic acid and pyruvic acid, has much greater thickening power than dextran and similar polysaccharides and hence can be used in significantly lower concentrations than the other materials. It is effective in both fresh water and brine and has excellent high temperature stability. It is not precipitated or adsorbed to a significant extent upon contact with porous rock and sands commonly found in oilbearing reservoirs. The combination of all of these properties makes the heteropolysaccharide formed by Xanthomonas considerably more attractive than other polysaccharides for use as water thickeners in enhanced recovery operations.
One difficulty that has been experienced with the use of Xanthomonas heteropolysaccharides as viscosifiers in flood waters is their tendency to plug the formations into which they are injected. This plugging problem is especially serious in oil reservoirs where salt concentrations in the water are high. These tendencies are generally thought to result from one or more of several factors; e.g. precipitation which occurs when solutions containing multivalent cations are injected into the formations containing alkaline materials; clumps of incompletely solubilized polymer; residual proteinaceous materials and/or residual whole bacterial cells or other cell debris from the fermentation process which produces the heteropolysaccharide.
A number of suggestions have been advanced to overcome the limitations with respect to the injectivity and/or the stability and clarity of these bipolymer solutions. For example, U.S. Pat. No. 3,355,447 to O'Connell, suggests heating an aqueous solution of the heteropolysaccharide under slightly alkaline conditions to moderate temperatures ranging up to 80.degree. C. for a period of time. This solution is subsequently cooled and filtered. This technique is essentially a pasteurization process which does show some benefit in decreasing the spoilage of heteropolysaccharide solutions over a period of time. However, the resulting solution does not have significantly improved injectivity characteristics.
U.S. Pat. No. 3,591,578 to Colin and Guibert, discloses another technique for improving the viscosity of aqueous solutions of heteropolysaccharide, especially solutions in hydrochloric acid medium and also solutions in various acid- or salt-containing media. In this technique, the broth is heated subsequent to fermentation to a temperature in the range of 80.degree. C. to 130.degree. C. for a period of 10 to 120 minutes at a pH of 6.3 to 6.9. The patent does not disclose that the heat-treated heteropolysaccharides have injectivity properties as hereinafter described and claimed.
A proposal for clarifying heteropolysaccharide solutions is disclosed in U.S. Pat. No. 3,711,462 to Abdo. In this technique bacterial debris is removed from aqueous solutions of the heteropolysaccharide material by contacting the solution with a montmorillonite clay to adsorb the debris. Mono- or divalent salts are added, if not already present in this solution, and a clay coagulant is mixed with this solution to flocculate the clay and adsorb debris which is ultimately removed from the clarified solution by mechanical means. This patented technique does improve the clarity of the heteropolysaccharide solution; however, injectivity is not significantly improved.
An approach to the problems of clarity and flowability is disclosed in U.S. Pat. No. 3,729,460 to Patton. In this technique the heteropolysaccharide is treated at elevated temperatures (up to 120.degree. C.) and under alkaline conditions, preferably, in a pH range from 11.2 to 12.8. It is disclosed that the technique improves the clarity and flowability of the heteropolysaccharide solution. However, it has been reported by Lipton that this patented technique does not completely solve the injectivity problem and the technique is reported to show very poor reproducibility (Lipton, "Improved Injectability of Biopolymer Solutions", SPE paper 5099, delivered at the 49th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, Houston, Texas, October 6-9, 1974).
A more recent development is disclosed in U.S. Pat. No. 3,801,502 to Hitzman where heteropolysaccharides useful as waterflood viscosifiers are prepared by adding 0.05 to 5 wt. % of at least one additive selected from the group consisting of alcohols, phenols, ketones and nonionic surfactants to the fermentation effluent containing the heteropolysaccharide followed by heating the same to increase the viscosity of the solution or dispersion containing the heteropolysaccharide to a desired level. Patentee states that the heteropolysaccharide can be dissolved or dispersed in water or brine and thereafter the resulting solution or dispersion is heated with an alcohol, ketone, phenol or nonionic surfactant. This patent, however, does not teach or suggest that a heteropolysaccharide having improved injectivity properties as herein provided can be obtained.
Accordingly, a genuine need exists for a method for producing a Xanthomonas heteropolysaccharide which can be readily filtered and separated from other debris in the solution such as bacterial cells and proteinaceous material. Moreover a need exists for a heteropolysaccharide which can be injected into porous media such an oil-bearing sandstone formations, and which can be added to foodstuffs or pharmaceutical preparations without discoloring or markedly changing the clarity of such preparations.